Process for hardening iron and steel alloys and article produced thereby



Patented oei. 20, 193

PATENT OFFICE PROCESS FOR HABDENING IRON AND STEEL ALLOYS AND ARTICLE PRO- DUCED THEREBY Marie Louis Andre Babinet, Boulogne-sur-Seine, France, assignor to The Nitralloy Corporation, a corporation-of Delaware No Drawing. Application December 1, 1933,

Germany Serial No. 700,571. In 1932 December 6,

10 Claims. 01. 148-4) This invention relates to a process of nitriding corrosion-resisting ferrous base alloys, and more particularly relates to the treatment of corrosionresisting ferrous base alloys containing chromium or chromium and nickel. The invention includes the nitrided articles of manufacture produced by the process.

The nitriding of corrosion-resisting ferrous alloys, such, for example, as corrosion-resisting chromium and chromium-nickel iron and steel alloys, offers diificulties because the chemical passivity of these alloys hinders the absorption of nitrogen into the surface layers. The passivity of corrosion-resisting iron and steel alloys can be removed by pickling with reducing acids, for example, with l-ICl. HCl has, however, the undesired property that it strongly attacks the corrosion-resisting iron and steel alloys, and as a consequence, the surfaces of articles made of such 20 alloys are easily roughened if they are treated with HCl.

' The invention has for an object to provide a process of nitriding corrosion-resisting iron and steel alloys in which articles made of such alloys 2 are rendered amenable to the nitriding treatment andare nitrided without attacking the surfaces of the article in such a way as to roughen them.

I have found that-the passivity of such alloys can be removed, at least sufficiently to render them readily susceptible of hardening by nitriding, by treatment with phosphoric acid or phosphates, for example, zinc phosphate, cadmium phosphate, bismuth phosphate and cobalt phosphate. The phosphates can be used either in a neiizltral solution or in a solution with phosphoric By way of example, the process may be practiced in the following manner:

An articleprepared from a steel alloy contain- 40 ing about18% chromium and 8% nickel is treated with a dilute phosphoric acid solution of zinc phosphate at room temperature, or, if desired, at a moderately elevated temperature, say at about 80 C. Owing to the fact that the phosphoric acid 45 does not attack the alloy to any substantial V extent, it is not necessary to wash the article after the treatment as is required when articles are pickled with HCl to remove their passivity. Furthermore, the treated article can be nitrided either immediately or after drying, and if necessary,

after extended storing. After subjecting treated articles of the above mentioned steel alloy to a nitriding treatment with ammonia gas for about 100 hours, they show a uniform surface hardness from about 1000 to 1050 Vickers-Brinell.

The treatment in preparation for nitriding of the corrosion-resisting iron and steel alloys with phosphoric acid and solutions of the phosphates has the advantage that the salt layer forming on the surface of the alloy prevents the alloy from recovering its passivity for several days, so that the nitriding does not need to be carried out immediately after the treatment. In place of the treatment of the corrosion-resisting iron and steel alloys with solutions of phosphates, the salt may be applied to the surfaces of the article to be nitrided in powdered form. The article may then be nitrided.

I claim:

1. The process of nitriding corrosion-resisting 15 ferrous alloys which comprises treating such an alloy with a compound containing the P04 radical, thereby removing the passivity of the alloy, and then nitriding the alloy.

2. The process of nitriding corrosion-resisting 20 ferrous alloys which comprises treating such an alloy with phosphoric acid, thereby removing the passivity: of the alloy, and then nitriding tire alloy.

3. The process of nitriding corrosion-resisting ferrous alloys which comprises treating such an alloy with zinc phosphate, thereby removing the passivity of the alloy, and then nitriding the alloy.

4. The process of nitriding corrosion-resisting ferrous alloys which comprises treating such an alloy with phosphoric acid and a phosphate, thereby removing'the passivity of the alloy, and then nitriding the alloy.

"5. The process of nitriding corrosion-resisting ferrous alloys which comprises treating such an alloy with a dilute'phosphoric acid solution of zinc phosphate, thereby removing the passivity of the alloy, and then nitriding the alloy.

'6. The process of nitriding corrosion-resisting ferrous alloys which comprises applying a phos-' phate in powdered form to the surfaces of the article to be nitrided, and then nitriding the alloy.

7. The process of nitriding corrosion-resisting ferrous alloys which comprises treating such an alloy with a phosphate, thereby removing the plalssivity of the alloy, and then nitriding the a oy.

8. An article of manufacture consisting of a normally passive corrosion-resisting ferrous base alloy and having smooth nitride hardened surface portions, said surface portions having been prepared by mechanically working them to the desired smoothness and shape, thereafter treating said surfaces with a compound containing the P04 radical and then nitriding.

9. An article of manufacture consisting of a normally passive corrosion-resisting chromium nickel ferrous base alloy and having smooth nitride hardened surface portions, said surface portions having been prepared by mechanical]! working them to the desired smoothness and shape, thereafter treating said surfaces with a compound 10 containing the P04 radical and then nitriding.

nitriding. v Mann: LOUIS ANDRE BABINET. 1o

10. An article of manufacture consisting of a ferrous base alloy containing about 18% chromium and about 8% nickel and having smooth nitride hardened surface portions, said surfaceportions having been prepared by mechanically 5 working them to the desired smoothness and shape, thereafter treating said surfaces with a compound containing the P04 radical and then 

